Comparison of Narrow-Band Reflectance Spectroscopy and

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 112:17–27 (2000)
Comparison of Narrow-Band Reflectance Spectroscopy and
Tristimulus Colorimetry for Measurements of Skin and Hair
Color in Persons of Different Biological Ancestry
MARK D. SHRIVER* AND ESTEBAN J. PARRA
Department of Anthropology, Pennsylvania State University, University
Park, Pennsylvania 16802
KEY WORDS
narrow-band spectrometer; tristimulus colorimeter;
skin and hair pigmentation
ABSTRACT
We have used two modern computerized handheld reflectometers, the Photovolt ColorWalk colorimeter (a tristimulus colorimeter; Photovolt,
UMM Electronics, Indianapolis, IN) and the DermaSpectrometer (a specialized
narrow-band reflectometer; Cortex Technology, Hadsund, Denmark), to compare
two methods for the objective determination of skin and hair color. These instruments both determine color by measuring the intensity of reflected light of
particular wavelengths. The Photovolt ColorWalk instrument does so by shining
a white light and sensing the intensity of the reflected light with a linear
photodiode array. The ColorWalk results can then be expressed in terms of
several standard color systems, most importantly, the Commission International
d’Eclairage (CIE) Lab system, in which any color can be described by three
values: L*, the lightness; a*, the amount of green or red; and b*, the amount of
yellow or blue. Instead of a white light and photodiodes, the DermaSpectrometer
uses two light-emitting diodes (LEDs), one green and one red, to illuminate a
surface, and then it records the intensity of the reflected light. The results of
these readings are expressed in terms of erythema (E) and melanin (M) indices.
We measured the unexposed skin of the inner upper arm, the exposed skin of the
forehead, and the hair, of 80 persons using these two instruments. Since it is
important for the application of these measures in anthropology that we understand their relationship across a number of different pigmentation levels, we
sampled persons from several different groups, namely, European Americans
(n ⫽ 55), African Americans (n ⫽ 9), South Asians (n ⫽ 7), and East Asians (n ⫽
9). In these subjects, there is a very high correlation between L* and the M index
for the inner arm (R2 ⫽ 0.928, P ⬍ 0.001), the forehead (R2 ⫽ 0.822, P ⬍ 0.001),
and the hair (R2 ⫽ 0.827, P ⬍ 0.001). The relationship between a* and the E
index is complex and dependent on the pigmentation level. We conclude that
while both types of instruments provide accurate estimates of pigment level in
skin and hair, measurements using narrow-band instruments may be less affected by the greater redness of certain body sites due to increased vascularization. Am J Phys Anthropol 112:17–27, 2000. © 2000 Wiley-Liss, Inc.
Over the past 50 years, skin pigmentation
levels have been objectively studied using
reflectance spectroscopy. The two instruments that have been most widely used in
anthropological studies are the E.E.L. instrument (Evans Electroselenium Co., Hal©
2000 WILEY-LISS, INC.
Grant sponsor: NSF; Grant number: 9610332.
*Correspondence to: Mark D. Shriver, Department of Anthropology, Pennsylvania State University, 409 Carpenter Building,
University Park, PA 16802. E-mail: [email protected]
Received 16 December 1998; accepted 13 January 2000.
18
M.D. SHRIVER AND E.J. PARRA
Fig. 1. A: Photovolt ColorWalk instrument being used to measure the inner arm of a subject (typically
the subject would be seated, as outlined in Subjects and Methods). B: DermaSpectometer, in a woman’s
hand.
stead, Essex, UK) and the Photovolt line of
instruments (UMM Electronics, Indianapolis, IN). Both of these instruments use colored filters to measure the percent reflectance of light of various wavelengths. The
E.E.L. instruments have a series of nine
colored filters, while the original Photovolt
systems have six available filters. Although
there is much data in the literature on pigmentation levels reported using these
E.E.L. and Photovolt filter-based reflectometers (reviewed in Robins, 1991), newer
technologies have led to smaller and more
accurate devices.
We compared two of these newer instruments for measurements of skin and hair
pigmentation, the Photovolt ColorWalk colorimeter (Photovolt, UMM Electronics, Indianapolis, IN) and the DermaSpectrometer
(Cortex Technology, Hadsund, Denmark).
These instruments apply two different re-
flectance technologies that have both been
widely used in the field of dermatology
(Takiwaki et al., 1994, Fullerton et al.,
1996). The ColorWalk is a handheld tristimulus colorimeter, which uses photodiode
arrays in lieu of colored filters to measure
the intensity of particular wavelengths of
light (see Fig. 1A). Tristimulus colorimetry
was developed as a means of objectively representing color in a manner analogous to
the way the eye perceives color (Hunter,
1942). The reflectance level of light through
three particular broad wavelength filters
(photodiode arrays on newer instruments) is
determined. Color parameters are then defined by the levels of and differences
among the reflectance levels of these three
filters. The most commonly used color parameters are the Commission International
d’Eclairage (CIE) L*a*b* system established in 1976. In the CIELab color system,
MEASUREMENT OF SKIN AND HAIR PIGMENTATION
any color can be represented by three variables: L*, the lightness-darkness axis; a*,
the red-green axis; and b*, the blue-yellow
axis, which can be plotted in three-dimensional space. Tristimulus colorimeters like
the ColorWalk and the commonly used Minolta Chroma Meter 200 and 300 series machines (Minolta Co., Osaka, Japan) are usually able to report color values for a number
of other color systems as well.
The DermaSpectrometer is a different
type of instrument that was developed specifically for measurements of skin pigments,
namely hemoglobin and melanin. The DermaSpectrometer (see Fig. 1B) and related
instruments, namely the Erythema/Melanin Meter (DiaStron, DiaStron Ltd., Hampshire, UK) and the Mexameter (Courage
Khazaka), are based on the work of Diffey et
al. (1984). Hemoglobin and melanin are the
principal pigments visible in the skin: hemoglobin in the blood of the capilaries in the
dermis, and melanin in the keratinocytes
and melanocytes of the epidermis. Hemoglobin and melanin both absorb much light at
the lower wavelengths, with hemoglobin
showing a large peak in the green wavelengths and then a sharp drop-off, absorbing
very little light in the red wavelengths,
which is why blood is red. Melanin, both in
vivo and in vitro, shows absorbance of light
of all wavelengths, essentially a flat line
sloping down from the lower wavelengths to
the higher wavelengths (Kollias and Baqer,
1985). Based on these differences in the
spectral curves of hemoglobin and melanin,
Diffey et al. (1984) suggested that the reflectance of narrow-band light in the red spectrum would yield reasonable estimates of
the melanin content of a persons skin, following the equation
M ⫽log 10 (1/% red reflectance).
The degree of skin redness or erythema can
be calculated by subtracting the absorbance
due to melanin from the absorbance of the
green filter and is calculated as
E ⫽ log 10 (1/% green reflectance)
⫺ log 10 (1/% red reflectance).
19
SUBJECTS AND METHODS
Subjects were seated for 5 min with their
arms at their sides prior to being measured.
While waiting to be measured, they were
asked a number of questions, namely, their
name, date and place of birth, biological ancestry and ethnicity, and whether they had
recently colored their hair. Eighty persons
participated in the study: 55 of European
ancestry (Europeans or European Americans), 9 of African ancestry (African or African Americans), 7 South Asians (India and
Pakistan), and 9 East Asians (China, Taiwan, Philippines, and Korea). All persons
were current residents of Pittsburgh, PA or
the surrounding areas, and all were measured during the second and third weeks of
August 1998. Before use, both instruments
were calibrated using the specific white and
black calibration standards supplied by the
manufacturers. Measurements were first
taken with the DermaSpectrometer of the
following sites in this order: inner upper
right arm, inner upper left arm, forehead,
and hair. Three measurements were taken
of each site, moving the measurement head
a few centimeters between measurements.
As has been suggested, care was taken not
to apply too much pressure on the measurement head of the DermaSpectrometer, since
doing so could occlude blood from the region
being measured (Fullerton et al., 1996).
Measurements of the hair were only
taken on those persons who said they did
not color or bleach their hair. For these measurements, we carefully pressed down the
hair throughout the parietal region, making
sure that the scalp was not visible, and then
applied the measurement head to this area.
As with the skin, three measurements were
taken in different areas around the parietal,
to get a better average of the total level of
hair pigmentation. After measuring with
the DermaSpectrometer, we measured the
same body sites in the same manner with
the Photovolt ColorWalk instrument (see
Fig. 1A for how a measurement is taken).
The ColorWalk allows the user to select
among the two commonly used reference illuminants and observation angles in tristimulus colorimetry. As previously suggested by Weatherall and Coombs (1992),
20
M.D. SHRIVER AND E.J. PARRA
TABLE 1. Summary of the results for melanin content (M and L*), and hemoglobin content (E and a*), in the
groups included in the present study1
M
L*
E
a*
1
African Americans
(n ⫽ 9)
Mean
s.d.
CV
East Asians
(n ⫽ 9)
Mean
s.d.
CV
European Americans
(n ⫽ 55)
Mean
s.d.
CV
South Asians
(n ⫽ 7)
Mean
s.d.
CV
56.62
47.50
2.68
11.17
31.79
67.30
6.92
12.46
30.50
69.86
6.64
12.72
37.13
61.90
6.81
13.25
14.78
9.95
5.08
2.25
26%
21%
190%
20%
2.39
1.62
0.83
1.21
8%
2%
12%
10%
2.82
3.26
1.20
1.89
9%
5%
18%
15%
4.19
3.76
0.75
0.77
11%
6%
11%
6%
s.d., standard deviation; CV, coefficient of variation (s.d./mean)*100.
we used the most recently established standards, namely an observation angle of 10°
and the D65 light source.
Both the ColorWalk and the DermaSpectrometer perform internal calculations to
convert the raw reflectance readings to the
output variables, namely the E and M indices for the DermaSpectrometer, and L*, a*,
and b*, in addition to other color systems for
the ColorWalk. Linear and nonlinear regression lines were calculated using standard statistical software packages.
RESULTS
Skin
In Table 1, we summarize the results obtained using the ColorWalk and the DermaSpectrometer for measuring the melanin
content (M and L*) and hemoglobin content
(E and a*) in a sample of 80 individuals
from different ethnic groups. Figure 2 shows
the relationship between the L* level, the
lightness in the CIELab color system, and
M, the melanin index, as measured at the
inner upper arm, for the 80 persons studied.
There is a clear relationship between these
two values: as L* decreases, indicating less
lightness and less reflectance, the M index
increases, indicating higher melanin content in the skin. The linear equation for this
line is
L* ⫽ 94.15 ⫺ 8132M
(R2 ⫽ 0.928, P ⬍ 0.001). However, it is apparent from the figure that the relationship
between L* and M is not strictly linear,
especially at high melanin concentrations,
and a slightly better fit is obtained using an
exponential equation (R2 ⫽ 0.962, Table 2).
Population differences in pigmentation level
are evident in Figure 2. The subjects of Eu-
ropean ancestry have the lightest skin, high
L*, and low M. Persons of East Asian ancestry have pigment levels which cluster at the
lower end of the European distribution. Persons of South Asian ancestry (Indian and
Pakistani) are the next darkest group and
overlap with some of the African-American
subjects, who have the darkest skin as well
as the widest variance in pigmentation
level.
Figure 3 shows the relationship between
L* and M for measurements of the forehead
of the 80 subjects studied. As with the measurements of the inner arm, there is a clear
correlation between L* and the M index
(R2 ⫽ 0.870, exponential regression, Table
2). It is also clear that the relationship between L* and M for the forehead is not as
strong as for measurements of the inner
upper arm.
Both a* and the E index have been used
by dermatologists as indicators of the degree of skin redness or erythema (Diffey et
al., 1984; Seitz and Whitmore, 1988; Serup
and Agner, 1990; Westerhof et al., 1990;
Takiwaki et al., 1994). Our data (not shown)
indicate that the relationship between a*
and E is complex, and dependent on the
level of pigmentation. There is a clear positive correlation between a* and E in persons
with low melanin content (M ⬍ 40). However, heavily pigmented persons (M ⬎ 40)
show a much lower correlation and a much
less steep relationship between a* and E
than lightly pigmented persons.
Given this complex relationship between
a* and E, it is important to understand both
how E varies with respect to M, and how a*
varies with respect to L*. Figures 4 and 5
show the results of these comparisons for
measurements of the inner upper arm.
MEASUREMENT OF SKIN AND HAIR PIGMENTATION
21
Fig. 2. Relationship between L* and the M index for the inner arm average of all persons measured.
L* was measured using the ColorWalk, and the M index using the Dermaspectrometer, as described in
Subjects and Methods. Also indicated is the biological ancestry of the persons measured: Europeans and
European Americans (open circle), East Asians (solid diamond), South Asians (⫻), and Africans and
African Americans (open square).
TABLE 2. Relationship between the parameters used
for estimating melanin content (M and L*), as
measured in the inner arm, forehead and hair
Inner arm
L* vs. M
M vs. L*
Forehead
L* vs. M
M vs. L*
Hair
L* vs. M
M vs. L*
Best fit
R2
L* ⫽ 110.1e⫺0.0151M
M ⫽ 300.53 ⫺ 63.646Ln(L*)
0.9617
0.9617
L* ⫽ 94.032e⫺0.0127M
M ⫽ 317.06 ⫺ 68.647Ln(L*)
0.8705
0.8705
L* ⫽ 167.45 ⫺ 30.494Ln(L*)
M ⫽ 222.8e⫺0.029L*
0.8852
0.8852
Again, the relationship between these parameters differs, depending on the level of
pigmentation, and there are important differences between the E vs. M plot (Fig. 4)
and the plot of a* and L* (Fig. 5). There is no
significant correlation between E and M in
the groups showing M values lower than 40
(low pigmentation levels, R2 ⫽ 0.0515, n.s.),
but in persons with M values higher than 40
(high pigmentation levels), a significant
negative correlation is observed (R2 ⫽ 0.976,
P ⬍ 0.001). On the contrary, the plot of L*
vs. a* shows a clear negative correlation in
the range corresponding to L* values higher
than 60 (low pigmentation levels, R2 ⫽
0.565, P ⬍ 0.001), and this correlation is not
observed in the region of L* values lower
than 60 (high pigmentation levels, R2 ⫽
0.0000, n.s.).
Figure 6 shows a histogram of the population distribution of the L* inner upper
arm measures for persons of European ancestry. We constructed this histogram using
a bin width of two L* units, as suggested by
Weatherall and Coombs (1992). The minimum and maximum L* values were 58.7
and 75.3, respectively, and the mode was
observed in the range of 71–73 (average L*,
69.9). The L* distribution is highly skewed
22
M.D. SHRIVER AND E.J. PARRA
Fig. 3. Relationship between L* and the M index for the forehead average of all persons measured.
L* was measured using the ColorWalk and the M index using the Dermaspectrometer as described in
Subjects and Methods. Also indicated is the biological ancestry of the persons measured: Europeans and
European Americans (open circle), East Asians (solid diamond), South Asians (⫻), and Africans and
African Americans (open square).
and very similar to another report from the
literature on the CIE color system in Europeans (Weatherall and Coombs, 1992). The
M index showed a very similar distribution,
but it was skewed to the right instead of the
left (data not shown). Both of these distributions are similar in shape to the log-normal distribution.
Hair
In addition to studies of the skin, reflectometers have been used to objectively
quantify the color and degree of pigmentation of the hair (Sunderland, 1956; Little
and Wolf, 1981). Figure 7 shows our results
for the measurement of the hair of the 64
persons in this survey who did not color or
bleach their hair. Figure 7 shows the relationship between L* and the M index for
these persons. There is a clear correlation
between the two measures L* and M (loga-
rithmic regression, R2 ⫽ 0.885, P ⬍ 0.001).
Notable is the limited variability in the hair
pigment level of non-European persons. Europeans demonstrate hair reflectance levels
that span the range of variation observed.
DISCUSSION
In the present study, we used two handheld reflectometers to compare two methods
for the determination of skin and hair color:
a narrow-band spectrometer (DermaSpectrometer) and a tristimulus colorimeter
(ColorWalk). We sampled persons of different biological ancestry in an effort to span
the range of variability in human pigmentation levels. Previous studies were performed by dermatologists in efforts to explore the relationships between the two
types of instruments that we consider in
this study (e.g., Takiwaki et al., 1994; Fullerton et al., 1996). One important consider-
MEASUREMENT OF SKIN AND HAIR PIGMENTATION
23
Fig. 4. Relationship between M index and E index for inner upper arm average. Biological ancestry
of the persons measured: Europeans and European Americans (open circle), East Asians (solid diamond),
Southwest Asians (⫻), and Africans and African Americans (open square)
ation regarding these studies is that the
focus in the dermatological literature has
been most often on the measurement of erythema, the reddening of the skin in response to irritation from ultraviolet light or
other causes; researchers have not considered the whole range of human pigmentation levels. In contrast, in the anthropological literature, the main focus has been an
objective determination of the melanin content of the skin (e.g., Korey, 1980; Relethford et al., 1983; Robins, 1991).
Melanin and hemoglobin are the two dominant chromophores of the skin. The melanin index, M (measured by means of a narrow-band spectrometer), and the L* value of
the CIELab space (measured by means of a
tristimulus colorimeter) have been used by
dermatologists as indicators of the melanin
content of the skin. A previous study of 10
individuals of Caucasian ancestry (Takiwaki et al., 1994) reported a moderate but
significant negative correlation between M
and L* (R2 ⫽ 0.314, P ⬍0.001). In our larger
sample of persons of European ancestry
(n ⫽ 55), we observed a higher correlation
(upper inner arm, R2 ⫽ 0.624, P ⬍0.001),
and this correlation is even higher when we
include individuals representative of all
ethnic groups (R2⫽0.928, P ⬍ 0.001). Both
L* and M seem to be highly correlated with
the melanin content of the skin, but the
melanin index (M), which has been specifically designed by taking into account the
absorbance spectrum of melanin and hemoglobin, may likely be a better indicator of
the melanin content than L*. The value of
L* is highly dependent on the reflected
green light, where in fact hemoglobin has its
peak absorption, so that the L* value is not
just a function of the melanin concentration.
This is clearly indicated by the significant
correlation observed between L* and a* in
low-pigmented persons (Fig. 5). The more
24
M.D. SHRIVER AND E.J. PARRA
Fig. 5. Relationship between L* and a* for inner upper arm average. Biological ancestry of the
persons measured: Europeans and European Americans (open circle), East Asians (solid diamond),
Southwest Asians (⫻), and Africans and African Americans (open square).
red the skin is, the lower the L* is for these
persons. This same trend was observed in
previous studies (Takiwaki et al., 1994, Fullerton et al., 1996, Takiwaki, 1998), and
may be responsible for the decreased correlation of L* and M in some comparisons
(e.g., forehead, where there is increased vascularization and sometimes quite dramatic
intraindividual variability).
Both a* and the E index have been used
by dermatologists as indicators of the degree of skin redness or erythema. A high
positive correlation has been observed between a* and E in a small sample of 10
Caucasian male volunteers, in whom measures were taken at 23 different anatomical
sites (R2 ⫽ 0.846, P ⬍ 0.001). We observed
in our sample of 55 persons of European
ancestry a lower, but still significant correlation (R2 ⫽ 0.379, P ⬍ 0.001). However,
this correlation is not significant in highly
pigmented persons (M ⬎ 40), indicating the
complex relationship between a* and E, and
the substantial differences in what both parameters are measuring. This is not surprising, if we take into account the different
methodologies upon which the DermaSpectrometer and the ColorWalk are based.
The comparisons of M vs. E and L* vs. a*
further stress these differences. M and E
are not correlated in low-pigmented groups
(Fig. 4). Thus, E is a good indicator of hemoglobin content in those groups, and behaves
independently of M. However, in groups
characterized by high melanin content,
there is a significant negative correlation
between E and M (R2 ⫽ 0.990, P ⬍ 0.001),
indicating that E is no longer a linear function of hemoglobin content. This is intrinsically due to the methodological principle
upon which the calculation of the melanin
and erythema indices is based. The melanin
index is calculated based on the amount of
red light reflected, given that hemoglobin
MEASUREMENT OF SKIN AND HAIR PIGMENTATION
Fig. 6.
25
Population distribution of inner upper arm L* levels for Europeans.
does not absorb in this spectrum, and consequently does not interfere in the calculation of the melanin content. On the contrary, both melanin and hemoglobin absorb
light in the green part of the spectrum, and
when high concentrations of melanin are
present, its effect on the amount of green
light reflected is substantial, and the E
value is no longer linearly related to the
hemoglobin content (Takiwaki et al., 1994).
When we consider the relationship between
L* and a*, exactly the opposite trend is observed. These values are highly correlated
when the amount of melanin is low, but
there is no significant correlation when the
melanin level is high (Fig. 5).
CONCLUSIONS
With recent technical advances in the
field of colorimetry and photometry, new instruments have become available which offer substantial advantages over previously
used instruments, in terms of precision,
portability, and ease of use. In this paper,
we used two new handheld reflectometers,
the Photovolt ColorWalk (a tristimulus colorimeter) and the DermaSpectrometer (a
specialized narrow-band reflectometer), to
compare two methods for the objective de-
termination of skin and hair color. Both instruments operate based on two different
principles. Our results indicate that both
types of instruments provide good and correlated estimates of pigment level in skin
and hair. However, we find that measurements using narrow-band instruments
(DermaSpectrometer, in this study) appear
to be less affected by the increased redness
of certain body sites due to increased vascularization. It is also evident that E and a*,
the parameters normally employed for evaluating the degree of erythema, show a complex relationship, which is dependent on the
melanin content of the skin.
It is necessary to point out that there are
a number of portable and handheld true
spectrophotometers currently available
(e.g., the spectrophotometer CM-500 and
CM-2000 series by Minolta, Japan, and the
Microflash series by Datacolor International, Charlotte, NC). Although somewhat
more expensive (2–3 times the cost of the
instruments we used in this study; $4,995
for the ColorWalk, and $4,500 for the DermaSpectrometer), these spectrophotometers are more versatile in that they measure the reflectance at regular intervals
across the spectrum of visible light (400 –
26
M.D. SHRIVER AND E.J. PARRA
Fig. 7. Relationship between the M index and L* for hair. Biological ancestry of the persons measured: Europeans and European Americans (open circle), East Asians (solid diamond), South Asians (⫻),
and Africans and African Americans (open square).
700 nm). Both have internal software,
which computes values for different color
systems, including CIELab. Since these instruments can display and record reflectance levels at narrow intervals across the
visual spectrum, one could also calculate
the E and M indices using these instruments. Additionally, having reflectance information across the whole visible spectrum
would make it possible to better distinguish
the effect of diverse skin chromophores
(melanin, oxy- and deoxy-hemoglobin, bilirubin), and even different forms of melanin
(high and low molecular weight melanin,
Kollias and Baqer, 1987, 1988; and potentially, eumelanin and pheomelanin).
In summary, both the DermaSpectrometer and the ColorWalk provide accurate
and objective measurements of skin and
hair color, which are highly correlated (Fullerton et al., 1996; Takiwaki, 1998). In anthropological and genetic studies where the
primary aim is the determination of skin
pigmentation due to melanin, the DermaSpectrometer would likely be the preferred instrument, as the M index obtained
with this apparatus is less confounded by
levels of hemoglobin and thus better reflects
the amount of melanin present in the skin.
This is especially true if comparisons of different body sites will be made (see also
Lock-Andersen and Wulf, 1998). In lightly
pigmented persons, differences in degree of
vascularization of different body sites make
the use of L* troublesome for these types of
comparisons.
ACKNOWLEDGMENTS
We thank all of the participants in this
study for their willingness and cooperation.
This study was funded in part by NSF grant
9610332 to M.S. We also thank Dr. Gary
Grove of cyberDERM, Inc. (Media, PA) for
the loan of the DermaSpectrometer, as well
MEASUREMENT OF SKIN AND HAIR PIGMENTATION
as the Editor and four reviewers for their
constructive comments on this manuscript.
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